Kinetics and mechanism on the epoxidation of cis-1-propenylphosphonic acid in H2O catalyzed by tungstate(VI) or molybdate(VI)

Journal of Molecular Catalysis A: Chemical 206 (2003) 213–223

Kinetics and mechanism on the epoxidation of cis-1-propenylphosphonic acid in H2 O catalyzed by tungstate(VI) or molybdate(VI) Xin-Yan Wang1 , Hong-Chang Shi∗ , Shou-Yi Xu Chemistry department of Tsinghua university, Beijing 100084, China Received 16 April 2003; received in revised form 14 June 2003; accepted 17 June 2003

Abstract Epoxidation of cis-1-propenylphosphonic acid (CPPA) was carried out by using Na2 WO4 or Na2 MoO4 as catalyst, 30% hydrogen peroxide as an oxidant in H2 O. Kinetic study showed that the epoxidation was zero-order both in H2 O2 and in CPPA with perfect linearity. The reaction was first-order only in catalysts. IR spectra showed that the active peroxo group in the epoxidation was a metal–dioxygen ring. The characteristic absorption peaks were 856 and 866 cm−1 for Mo– and W–dioxygen rings, respectively. A mechanism of the epoxidation was suggested and discussed. The pH value of reaction solution and the negative charge on the carbon atoms of double bonds play an important role because they could influence the concentration of active complexes and the rate of insertion reaction. The research indicated that not only in non-protic solvents but also in H2 O, the active peroxo groups were the metal–dioxygen rings for the epoxidation of olefins catalyzed by W(VI) or Mo(VI) complexes. © 2003 Elsevier B.V. All rights reserved. Keywords: Na2 WO4 ; Na2 MoO4 ; Epoxidation; Mechanism; cis-1-Propenylphosphonic acid (CPPA)

1. Introduction Many ␣,␤-unsaturated acids, such as maleic acid, fumaric acid, citraconic acid and cis-1-propenylphosphonic acid (CPPA) can easily dissolve in water because they have strong hydrophilic groups, such as carboxyl, phosphonic group in the molecules. For this reason, their epoxidation can carry out in water [1]. (−)(1R,2S)-1,2-epoxypropyl phosphonates (Fosfomycin) is a broad-spectrum antibiotic [2], which can be obtained by the epoxidation of cis-1-propenylphosphonic acid (CPPA) and the res∗ Corresponding author. E-mail addresses: [email protected] (X.-Y. Wang), [email protected] (H.-C. Shi). 1 Tel.: +86-1-62783878; fax: +86-1-62771149.

olution of the racemic products. We have carried on many epoxidation experiments of CPPA in H2 O without any catalyst present but no epoxide was obtained. However, when Na2 WO4 or Na2 MoO4 was used as catalysts, the epoxidation could be completed easily. The mechanism study of the epoxidation of olefins was mainly aimed at non-protic solvents before [3,4]. Therefore, the mechanism of CCPA epoxidation in H2 O is an interesting problem. Mimoun has summarized as two paths concerning the mechanism of W(VI) or Mo(VI) complexes catalyzed epoxidation of olefins in non-protic solvents [5]. The mechanisms were shown in Scheme 1(a) and (b). In path (a), the active peroxo group was a metal– dioxygen ring 2. The peroxo complex behave as a 1,3-dipolar reagent M+ –O–O− [5]. The key active

1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1381-1169(03)00460-6

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no-protic solution O

(a) M

O

H2O2

M

M

O

4 O

O 6

O +

9 O 1

M

2 O

1

(b) M ROOH

O

O

M 1

R

O M

O

M

3

O O 5 O

M

O O R 7

R O

8

M R

O +

O M

R

9

Scheme 1. Mechanism of W(VI) or Mo(VI) catalyzed epoxidation of olefins in non-protic solution.

intermediate was the metal–olefin–dioxacycle complex 4. The olefin in 4 inserted into the metal–oxygen bond and formed a closed five-membered dioxametalcycle adduct 6. In path (b), the complexation of metal center with ROOH and olefin formed an acyclic metal–olefin– peroxo complex 5. The insertion of the olefins into the M–O bond via intra-molecular nucleophilic attack of the –OOH or –OOR group at the coordinated olefin would lead to the formation of an acyclic adduct and leave a vacant coordination site on the metal 7. A quasi-opened, five-membered dioxyethylmetal adduct 8 was formed from electron donation of peroxo oxygen. Mimoun [5], and Chong and Sharpless [6] considered that M–OOR was the active peroxo group in the path while Mimoun also pointed out that the active intermediates in the path have not yet been fully characterized. Our objective of the research is to make clear the epoxidation mechanism of CPPA in H2 O catalyzed by tungstate(VI) or molybdate(VI).

2. Results and discussion CPPA is a strong acid, it must be neutralized to pH 5.0–6.0 by an organic or inorganic base (such as NaOH) before epoxidation. Among organic bases,

racemic ␣-phenylethylamine was used often, while it was also used as resolution reagent producing Fosfomycin [7]. The epoxidation is shown in Scheme 2. The kinetic study showed that the epoxidation was zero-order both in H2 O2 and in CPPA with perfect linearity (Figs. 1 and 2). It was first-order only in catalysts (Fig. 3). The rate constants are shown in Table 1. As shown in Table 1, the reaction rate was independent of the concentration of the two reactants and was only dependent on the concentration of the catalyst. It was indicated the process involved a rapid irreversible metal–olefin–peroxo complex formation from the catalysts, CPPA and H2 O2 in H2 O. The concentration of the complex was precisely equal to that of catalyst. M–OOH should be first formed in the reaction of tungstate(VI) or molybdate(VI) with H2 O2 . Therefore, if M–OOH were really the active peroxo group, the epoxidation of CPPA would be carried out through path (b) in Scheme 1 to form metal–olefin–peroxo complex 5 rapidly. However, IR spectra showed that the metal–dioxygen ring complexes (2 in Scheme 1) were formed in H2 O and displayed strong activity of epoxidation. The solution of Na2 MoO4 (2.05 mol l−1 ) and Na2 WO4 (2.05 mol l−1 ) were neutralized to pH 5.5 by adding equal molar hydrochloric acid (a1 , b1 in Fig. 4). When equal moles 30% of H2 O2 was added, the solution immediately turned into yellow that was

X.-Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 213–223

H3C

PO3 H2

H

H3C O PO3- HQ+ Na2MO4 H H2O2 H

H3C

H

H

215

PO3-HQ+ H

M = W or Mo +

Q+= H3 N CH CH3 or Na+ Ph Scheme 2. Epoxidation of CPPA catalyzed by Na2 WO4 or Na2 MoO4 in H2 O.

0.10

-1

C0-C (mol ·L )

0.08

0.06

0.04

0.02

0.00 0

10

20

(a)

30

40

50

t (min)

0.09 0.08 0.07

-1

C0-C (mol·L )

0.06 0.05 0.04 0.03 0.02 0.01 0.00 0

(b)

20

40

60

80

100

120

t (min)

Fig. 1. Zero-order plot in hydrogen peroxide for the epoxidation catalyzed by Na2 WO4 and Na2 MoO4 in H2 O at different temperatures: (a) Na2 WO4 ; (b) Na2 MoO4 . Molar concentrations (mol l−1 ): CPPA, 1.36; H2 O2 , 0.087; Na2 WO4 , 0.61 × 10−3 ; Na2 MoO4 , 1.62 × 10−3 . Reaction temperatures (±0.1 ◦ C): (a) (䊏) 30, (䊉) 35, (䉱) 40; (b) (䊏) 30, (䊉) 40, (䉱) 50.

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0.16 0.14

-1

C0-C (mol·L )

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0

10

20

30

40

(a)

50

60

70

80

t (min)

0.14

0.12

-1

C0-C (mol·L )

0.10

0.08

0.06

0.04

0.02

0.00 0

20

40

60

80

(b)

100

120

140

160

180

200

t (min)

Fig. 2. Zero-order plot in CPPA for the epoxidation catalyzed by Na2 WO4 and Na2 MoO4 in H2 O at different temperatures: (a) Na2 WO4 ; (b) Na2 MoO4 . Molar concentrations (mol l−1 ): CPPA, 0.174; hydrogen peroxide,1.131; sodium tungstate, 0.78 × 10−3 ; sodium molybdate, 2.06 × 10−3 . Reaction temperatures (±0.1 ◦ C): (a) (䊏) 30, (䊉) 35, (䉱) 40; (b) (䊏) 30, (䊉) 40, (䉱) 50.

the characteristic color of metal (Mo or W)–dioxygen ring. The characteristic absorption peaks were 856 and 866 cm−1 for Mo– and W–dioxygen rings, respectively (a2 , b2 in Fig. 4). The temperature of the above solution was dropped to 10 ◦ C and then equal moles of CPPA solution (neutralized to pH 5.0 by equal moles of NaOH) was added. The epoxidation

occurred immediately and the temperature of the mixture goes up soon. 1 H NMR showed that the reaction was completed in 10 min. The absorption peaks at 856 and 866 cm−1 in IR spectra disappeared (a3 , b3 in Fig. 4). The IR spectra indicated that in the epoxidation of CPPA the active peroxo group was the metal–dioxygen

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217

110 100 90

r (mol·L-1·min-1)

80 70 60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

-1

Catalyst concentration (mol·L ) Fig. 3. First-order plot in catalyst for the epoxidation at 40 ◦ C catalyzed by: (a) (䊏) Na2 WO4 ; (b) (䊉) Na2 MoO4 . Molar concentrations (mol l−1 ): CPPA, 1.36; Na2 WO4 , (6.10–3.57) × 10−4 ; Na2 MoO4 , (0.81–6.48) × 10−3 ; H2 O2 , 8.65 × 10−2 .

ring 2, not M–OOR 3 and the active intermediate was 4, not 5 (see Scheme 1). Therefore, we suggested a mechanism for the epoxidation of CPPA catalyzed by Na2 WO4 or Na2 MoO4 in H2 O as shown in Scheme 3. The mechanism was actually the combination of Scheme 1(a) and (b) because 12 in Scheme 3 (5 in Scheme 1) was formed first though it was not a active peroxo complex and then it could be transformed into the active peroxo complex 13 (4 in Scheme 1).

(1) The solution was alkaline (pH 7.5–8.0) when Na2 WO4 or Na2 MoO4 was dissolved in H2 O. Equal moles of hydrochloric acid was added to neutralize the solution to pH 5.5, so tungstate and molybdate mainly existed in the form of MO4 H− at pH 5.0–6.0 (10 in Scheme 3). CPPA could be neutralized to pH 5.0 by adding equal moles of ␣-phenylethylamine or NaOH to the solution. Therefore, CPPA mainly existed in the

Table 1 Rate constants for the epoxidation of CPPA catalyzed by Na2 WO4 or Na2 MoO4 Temperature (◦ C)

30 35 40 50

k × 104 (mol (l min)−1 )a

k × 104 (mol (l min)−1 )b

k × 104 (min−1 )c

Na2 WO4

Na2 WO4

Na2 WO4

19.50 ± 0.10 27.20 ± 0.45 35.60 ± 0.33

Na2 MoO4 6.20 ± 0.08 18.50 ± 0.09 33.40 ± 0.50

18.80 ± 0.09 25.60 ± 0.34 34.40 ± 0.25

Na2 MoO4 7.20 ± 0.11 17.00 ± 0.21 34.70 ± 0.54

Na2 MoO4

29.20 ± 1.04 11.78 ± 0.22

a Zero-order rate constants in H O (mol l−1 ): [CPPA] = 1.36; [Na WO ] = 6.10 × 10−4 ; [Na MoO ] = 1.62 × 10−3 ; [H O ] = 2 2 2 4 2 4 2 2 8.65 × 10−2 . b Zero-order rate constants in CPPA (mol l−1 ): [CPPA] = 0.17; [Na WO ] = 7.77 × 10−4 ; [Na MoO ] = 2.06 × 10−3 ; [H O ] = 1.13. 2 4 2 4 2 2 c First-order rate constants in catalyst (mol l−1 ): [CPPA] = 1.36; [Na WO ] = (6.10–3.57) × 10−4 ; [Na MoO ] = (0.81–6.48) × 10−3 ; 2 4 2 4 [H2 O2 ] = 8.65 × 10−2 .

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a1

836

890

a2 960

n (o-o) 890 856

mono-negative ionic form (CH3 CH=CHPO3 − H, 12–15 in Scheme 3) at pH 5.0–6.0. (2) The epoxidation was zero-order both in H2 O2 and in CPPA. This indicates that the formation of 10 and 11 were rapid. The equilibrium was present between non-active peroxo complex 12 and active peroxo complex 13. It was obviously that the reaction was first-order in catalyst. K = [13]/[12] r = k[13] = kK[12] = k [catalyst]

a3

913 983

1200

(a)

1100

843

1000

900

800 -1

Wavenumbers (cm )

b1

946

896 840 866

833

b2

n (o-o)

953

b3 983 946 1073 1200

(b)

1100

1000

900

800 -1

Wavenumbers (cm )

Fig. 4. IR spectra of Na2 WO4 and Na2 MoO4 in H2 O (measured with Spectrum GX, FT–IR system, made by PE company. Horizontal ATR affix of ZnSe crystal was used, 32 times scanning with resolution of 4 cm−1 ). (a1 ) Na2 MoO4 (2.05 mol l−1 , pH 5.5); (a2 ) equal moles of H2 O2 was added; (a3 ) equal moles of CPPA (pH 5.0) was added at 10 ◦ C and the reaction was completed. (b1 ) Na2 WO4 (2.05 mol l−1 , pH 5.5); (b2 ) equal moles of H2 O2 was added; (b3 ) equal moles of CPPA (pH 5.0) was added at 10 ◦ C and the reaction was completed.

The concentration of catalyst was usually very low, thus the concentration of 13 was much lower. Although 13 was very active, the epoxidation was slow below room temperature because of the very low concentration of 13. The concentration of 13 would be increased when the reaction temperature was raised. It means that K was increased. Therefore, the epoxidation was usually carried out at 50–60 ◦ C. The epoxidation of CPPA in H2 O has excellent retention of configuration (cis-CPPA can produce 100% cis-epoxide). According to the mechanism shown in Schemes 1(a) and 3, the retention of configuration could be realized because the active group was metal–dioxygen ring. The olefin inserted into M–O bond through a 1,3-dipolar cycloaddition and the process should be concerted. If 12 or 5 are active intermediates, –O–O–R would fall off from the metal center as a whole and then connected to a carbon of the double bond. It was difficult to guarantee the concertedness of the process and 100% retention of configuration, especially at high reaction temperature. (3) Sharpless et al. [8] proved that the oxygen atom in epoxide could only come from M–O–O–R according to the experiments of 18 O labeled epoxidation of olefins. However, it could not yet be made sure that the M–OOR is an active peroxo complex. We speculated the M–OOR would be transformed into the active metal–dioxygen ring complex through the process shown in Scheme 4. Our experiments showed when Na2 WO4 or Na2 MoO4 was used as catalyst (catalyst:CPPA = 1:50), H2 O2 or AcOOH as oxidant, the epoxidation of CPPA could be completed in 0.5 h at

X.-Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 213–223

219

sodium molybdate in H2O O

-

O

HO

O

-

M

O

O O H OH

O

H 2O 2

M

HO

rapid 10

PO3-H

H3C H

H

rapid

-

O O

HO H H-O3P

11 -O

K

HO H

-

O

M

O H

O O

HO

M H

H-O3P

CH3

13

HO

12

O

H-O3P

-O

M

O O H OH H CH3

O O H CH3

14

O

O

O

M

H H-O3P

15

H

O H

H

H-O3P

+ 10

CH3

CH3

16

Scheme 3. Mechanism of epoxidation of CPPA catalyzed by sodium tungstate or sodium molybdate in H2 O.

50 ◦ C. However, if t-Bu–OOH was used as oxidant, the epoxidation did not occur with in 24 h under 100 ◦ C. Scheme 4 indicated that the R–O bond must be disconnected to form the active metal–dioxygen ring peroxo complex. If R was Hor acyl-, the disconnection was easy, and so the active metal–dioxygen ring complex formed easily. However, the disconnection of t-Bu–O bond was difficult which lead the active metal–dioxygen ring peroxo complex difficult to form. Therefore, Chong and Sharpless’s 18 O labeled epoxidation of olefins catalyzed by Mo(CO)6 with t-Bu–OOH cost 24 h at 115 ◦ C [6]. While in the molybdenum(VI) catalyzed epoxidation of propylene with t-Bu–OOH (Halcon process) [9], the epoxidation was carried out at 130 ◦ C for 2 h! It indicated

*O

ROOH

M

O M *O H

O

O M

R

O

+

*O R H

R=H, Acyl, Alkyl Scheme 4.

18 O

labeled process of the epoxidation.

t-butyl hydroperoxide as an oxidant is unfavorable to form the active metal–dioxygen ring peroxo complex, and so reaction temperature must be raised greatly. It means also that M–OOH was not an active peroxo group in the epoxidation. (4) The previous workers have already considered the orientation of the olefin molecule in the transition metals catalyzed insertion reaction. They concluded that for a polar double bond, the carbon atom bearing larger negative charge would attach to the metal center to form a new metal–carbon bond. Such mode was defined as “Markownikoff mode” of insertion [10]. We have also found that when a carbon atom of the double bond has high negative charge, the epoxidation could often be carried out using Na2 WO4 and Na2 MoO4 as catalysts. CPPA is an olefin having high negative charge on a carbon of the double bond. The calculation by semi-empirical quantum chemical method PM3 showed that a carbon has high negative charge (Table 2, 17). Thus, the carbon can combine easily with metal center that has positive charge and then the insertion reaction can easily occur (transition state 14 in Scheme 3). In Table 2, a carbon on double bonds of 18–20

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Table 2 Influence of the negative charges on the carbons of double bonds to the catalytic activity of Na2 WO4 Olefina H -0.14

17 H3C -

OOC -0.07

18 H H

19

-

-0.52

20 H

PO3H -

Temperature (◦ C)

Reaction time (h)

Yield (%)

H2 O

5.5

50

0.5

100b

H2 O

4–5.5

63–65

1.5

95c

H2 O

4–5.5

63–65

3

86c

H2 O

4–5.5

63–65

1

80c

COOH

COOH

-0.05

-0.25 H

H3C -0.04

pH

H

-0.23 H

OOC

Solvent

H

-0.20 COOH

a

PM3 method in Hyperchem. 6.0 Demo software package was used for calculating the charges on the carbon atoms of the double bonds. The yield of 17 was determined by 1 H NMR; Na2 WO4 :CPPA = 1:50. c The yields of 18–20 were reported by Payne and Williams [1]; Na WO :18 = 1:50; Na WO :19 = 1:10; Na WO :20 = 1:10. 2 4 2 4 2 4 b

also has high negative charges, and so all of them could be epoxidized by using Na2 WO4 as catalyst [1]. However, their epoxidation rate was much slower than that of CPPA. The reason originated probably from their lower negative charges on the carbon of double bonds than CPPA. Therefore, a carbon with double bond has high negative charge is favorable to the tungstate or molybdate catalyzed epoxidation. (5) The pH of the solution was an important factor influencing the reaction rate. The rate constants of the epoxidation of CPPA catalyzed by Na2 WO4 or Na2 MoO4 at 40 ◦ C under different pH values were shown in Table 3. The suitable pH range

of Na2 WO4 was 4.0–6.5. As for Na2 MoO4 , the suitable pH range was 5.0–6.5. The pH value of 5.5 was proved to be most suitable for the reaction. The epoxidation rates were decreased soon if the pH was in excess above the ranges. The H+ concentration has mainly influenced the equilibrium constant K between 12 and 13 (Scheme 5). The concentration of 13 should be largest at pH 5.0–6.0 and then the epoxidation rate was most rapid. Lower or higher pH values would decrease the concentration and the reaction rate was decreased. In addition, CPPA, Na2 WO4 and Na2 MoO4 in H2 O have the following equilibriums (Scheme 6). It was indicated that the active complex 13 has many and varied ionic structures with different activity under different pH value. For example,

Table 3 Rate constants for epoxidation of CPPA at different pH value at 40 ◦ C pH

k × 104 (mol (l min)−1 ) Na2 WO4

3.0 4.0 5.5 6.5 7.0

32.20 52.00 60.10 55.40 29.70

± ± ± ± ±

H+

Na2 MoO4 0.38 1.15 0.63 0.68 0.53

9.47 11.20 16.10 14.80 12.30

± ± ± ± ±

0.05 0.17 0.5 0.31 0.35

[CPPA] = 1.58 mol l−1 ; [Na2 WO4 ] = 7.05 × 10−4 mol l−1 ; [Na2 MoO4 ] = 1.92 × 10−3 mol l−1 ; [H2 O2 ] = 10.50 × 10−2 mol l−1 .

H O -O

O

M

H -

H O3 P

O

O

OH H H CH3

12

K

H H+ H O O O O M O HO H H H-O 3P

CH3

13

Scheme 5. The influence of H+ concentration.

X.-Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 213–223

H 2-O P 3

H

H+

H-O3 P

CH3

MO42-

H

H+

H

H+

CH3

MO4 -H

H+

H

H H2O3P

221

CH3

MO4H2

M=W, Mo Scheme 6. The ionic equilibrium of CPPA, Na2 WO4 and Na2 MoO4 in H2 O.

when pH >6.0, more CPPA ions having two negative charge (CH3 CH=CHPO3 2− ) would be formed and decreased the negative charge quantity on the carbon atom with the double bond. The calculation using PM3 method showed that the negative charge was turned into −0.39 on the carbon from −0.52 (17 in Table 2). It was unfavorable to the insertion of double bond in the epoxidation. (6) Phosphoric and phosphonic acids are known to form complexes with tungstic and molybdic anions in their peroxo- and non-peroxo forms. Recent investigation [11] indicated that some complexes are very efficient catalysts for the epoxidation of alkenes, by far more efficient than the uncomplexed Mo or W anions. Having a large number of P–O–W or P–O–Mo bonds is an important structural feature of the complexes. Therefore, if the complexes were formed, 31 P chemical shifts must be changed greatly [11a]. However, 31 P NMR spectra indicated that phosphonate (–PO3 − H) of CPPA did not have complexes with tungstate or molybdate because no other new 31 P chemical shift peak appeared except 31 P peak of epoxide in the course of the epoxidation. Furthermore, the formation of P–O–W or P–O–Mo bonds in our reaction system is difficult because this situation requires dehydrate in H2 O (it needs to eliminate one molecule of H2 O to form a P–O–W or P–O–Mo). Kinetic study also has been done in order to determine the phosponate in CPPA molecule through forming complex with catalysts to influence the reaction. We added NaH2 PO3 (equal molar weight of catalyst) to the reaction system. If the influence existed, the reaction rate should change. However, no change has been observed. It was indicated that phosponate

group did not participate in the epoxidation. Therefore, Na2 WO4 or Na2 MoO4 play independently catalysis role in the epoxidation of CPPA in H2 O. 3. Conclusion It was the first time we specialized in the kinetics and mechanism of the epoxidation of CPPA in H2 O catalyzed by Na2 WO4 or Na2 MoO4 . The research indicated that not only in non-protic solvents but also in H2 O, the active peroxo groups were the metal-dioxygen rings formed by complexing of W(VI) or Mo(VI) compounds with H2 O2 . Some results on kinetics and mechanism may have general significance for the epoxidation of some olefins such as ␣,␤-unsaturated acids by using W(VI) or Mo(VI) compounds as catalysts in H2 O. 4. Experiments Unless otherwise specified, all reagents were purchased from commercial suppliers and were used without purification. CPPA was 83% in content including impurities of propylphosphonic acid (CH3 CH2 CH2 PO3 H2 , about 7%), phosphonic acid (H3 PO4 , about 6%) and ethanol (about 2–3%). We did not purify the crude CPPA because the impurities have no obvious influence on the reaction. The temperature was controlled by thermostat with variation of ±0.1 ◦ C. 1 H NMR spectra were recorded on a Bruker AC 300 MHz NMR spectrometer using D2 O as solvent and were referenced to Me4 Si. IR spectra were measured with Spectrum GX, FT–IR system, made by PE Company. Horizontal ATR affix of ZnSe crystal was used, 32 times scanned with resolution of 4 cm−1 . The reproducibility of our experiments was

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very good and the data in the present work were the average values of 2–3 runs. 4.1. Kinetics in hydrogen peroxide CPPA (5 g, 0.04 mol) was dissolved in H2 O (15 ml) at room temperature. (±)␣-Phenylethylamine (5 g, 0.04 mol) was added to neutralize the solution to pH 5.5. To the solution was added sodium tungstate (0.005 g, 1.52 × 10−5 mol, dissolved in 1 ml H2 O) or sodium molybdate (0.01 g, 4.13×10−5 mol, dissolved in 1 ml H2 O). The reaction flask loaded with the mixture was then moved to thermostat. After stirring for half an hour, the mixture was kept at a certain temperature. Hydrogen peroxide (0.25 ml, 30% in aqueous, 2.43 × 10−3 mol) was then added to the solution. The reactions were carried out at several invariable temperatures during which aliquots were taken out at intervals for the iodometric titration analysis to determine the concentration of hydrogen peroxide. The analytical method was also used in experiments below.

mixture was kept at a certain temperature. Hydrogen peroxide (0.25 ml, 30% in aqueous, 2.43 × 10−3 mol) was then added to the solution. The following steps were the same as above. 4.4. Kinetic studies at different pH value CPPA (5 g, 0.04 mol) was dissolved in H2 O (20 ml) at room temperature. Sodium hydroxide ((g): 1.00; 1.20; 1.50; 1.60; 2.50; 2.90) was added to neutralize the solution to pH 2.5, 3.0, 4.0, 5.5, 6.5 and 7.0, respectively, followed by addition of sodium tungstate (0.005 g, 1.52 × 10−5 mol, dissolved in 1 ml H2 O) or sodium molybdate (0.01 g, 4.13×10−5 mol, dissolved in 1 ml H2 O). The reaction flask loaded with the mixture was then moved to thermostat. After stirring for half an hour, the mixture was kept at a certain temperature. Hydrogen peroxide (0.5 ml, 30% in aqueous, 2.43 × 10−3 mol) was then added to the solution. The following steps were the same as above. 4.5. IR spectra

4.2. Kinetics in CPPA CPPA (0.5 g, 4.09×10−3 mol) was dissolved in H

2O (15 ml) at room temperature. (±)␣-Phenylethylamine (0.5 g, 4.06 × 10−3 mol) was added to neutralize the solution to pH 5.5 followed by addition of sodium tungstate (0.005 g, 1.52 × 10−5 mol, dissolved in 1 ml H2 O) or sodium molybdate (0.01 g, 4.13 × 10−5 mol, dissolved in 1 ml H2 O). The reaction flask loaded with the mixture was then moved to thermostat. After stirring for half an hour, the mixture was kept at a certain temperature. Hydrogen peroxide (2.5 ml, 30% in aqueous, 2.43 × 10−2 mol) was then added to the solution. The following steps were the same as above.

4.3. Kinetics in catalysts CPPA (5 g, 0.04 mol) was dissolved in H2 O (15 ml) at room temperature. (±)␣-Phenylethylamine (5 g, 0.04 mol) was added to neutralize the solution to pH 5.5 followed by addition of sodium tungstate (0.005–0.03 g, (1.52–9.09) × 10−5 mol l−1 , dissolved in 1 ml water) or sodium molybdate (0.005–0.04 g, (2.07–16.5) × 10−5 mol, dissolved in 1 ml H2 O). The reaction flask loaded with the mixture was then moved to thermostat. After stirring for half an hour, the

The solution of Na2 MoO4 (2.00 g, 8.26×10−3 mol, solved in 4 ml H2 O) and Na2 WO4 (2.73 g, 8.26 × 10−3 mol, solved in 4 ml H2 O) were neutralized to pH 5.5 by adding equal moles of hydrochloric acid. After that, H2 O2 (0.85 ml, 30% in aqueous, 8.26 × 10−3 mol) was added. The temperature of the above solution was dropped to 10 ◦ C and then CPPA solution (1.01 g, 8.26 × 10−3 mol, neutralized to pH 5.5 by equal moles of NaOH) was added. References [1] G.B. Payne, P.H. Williams, J. Org. Chem. 24 (1959) 54. [2] British Pharmacopoeia, The Stationery Office, London, vol. 1, 2000, p. 729. [3] (a) H. Mimoun, I. Seree de Roch, L. Sajus, Tetrahedron 26 (1970) 37; (b) K.B. Sharpless, T.C. Flood, J. Am. Chem. Soc. 93 (1971) 2316; (c) T.G. Traylor, F. Xu, J. Am. Chem. Soc. 109 (1987) 6201; (d) A.K. Awasthy, J. Rocek, J. Am. Chem. Soc. 91 (1969) 991; (e) M.N. Sheng, J.G. Zajacek, J. Org. Chem. 35 (1970) 1839; (f) E.S. Gould, R.R. Hiatt, K.C. Irwin, J. Am. Chem. Soc. 90 (1968) 4573; (g) P.N. Balasubramanian, A. Sinha, T.C. Bruice, J. Am. Chem. Soc. 109 (1987) 1456;

X.-Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 213–223 (h) G.R. Howe, R.R. Hiatt, J. Org. Chem. 36 (1971) 2493; (i) T.N. Baker, G.J. Mains, M.N. Sheng, J.G. Zajacek, J. Org. Chem. 38 (1973) 1145. [4] (a) K. Sato, M. Aoki, M. Ogawa, T. Hashimoto, R. Noyori, J. Org. Chem. 61 (1996) 8310; (b) M. Gahagan, A. Iraqi, D.C. Cupertino, R.K. Mackie, D.J. Cole-Hamilton, J. Chem. Soc., Chem. Commun. 1989, p. 1688; (c) W.R. Thiel, M. Angstl, N. Hansen, J. Mol. Catal. Part A: Chem. 103 (1995) 5. [5] (a) H. Mimoun, Angew. Chem. Int. Ed. Engl. 21 (1982) 734; (b) H. Mimoun, J. Mol. Catal. 7 (1980) 1. [6] A.O. Chong, K.B. Sharpless, J. Org. Chem. 42 (1977) 1587.

223

[7] (a) M. Sletzinger, S. Karady, Ger. Offen. 1 (1970) 924,155; (b) E.F. Schoenewaldt, Ger. Offen. 1 (1970) 924,156. [8] K.B. Sharpless, J.M. Townsend, D.R. Williams, J. Am. Chem. Soc. 94 (1972) 295. [9] J. Kollar, US 3,360,584 (1967). [10] G.H. Olive, S. Olive, Coordination and Catalysis, Verlag Chemie, Weinheim, 1977, p. 122. [11] (a) G. Gelbard, F. Raison, E. Roditi-Lachter, R. Thouvenot, L. Ouahab, D. Grandjean, J. Mol. Catal. 114 (1996) 77; (b) K. Sato, M. Aoki, M. Ogawa, T. Hashimosto, R, Noyori, J. Org. Chem. 61 (1996) 8310; (c) Z.W. Xi, N. Zhou, Y. Sun, K.L. Li, Science 292 (2001) 1139.